1. Field of the Invention
The present invention relates to a driving force transmitting apparatus using a spring clutch adapted to be used with a sheet feeding system and the like.
2. Related Background Art
In the past, since spring clutches have simple constructions and are inexpensive, they have been used as driving force transmitting apparatuses with various systems in various technical fields.
Under the circumstances, in an image forming system such as a copying machine, laser beam printer or the like, a spring clutch has been used for permitting and prohibiting the transmission of a driving force from a motor to a supply roller for supplying or feeding out sheets one by one from a cassette in which a plurality of sheets are stacked.
For example, as shown in FIG. 10, in a sheet feeding system adopted to a laser beam printer, a sheet material S fed by a supply roller 50a is guided by guide plates 52a and 52b disposed at a downstream side with respect to a sheet feeding direction (referred to as merely "downstream side" hereinafter) until it reaches temporarily stopped regist rollers 53, where the skew-feed of the sheet is corrected by abutting a leading end of the sheet against the regist rollers. Then, by rotating the regist rollers, the sheet S is fed to a recording portion of the printer.
As shown in FIGS. 11A, 11B and 12, a spring clutch is attached to one end of a rotary shaft 51 of the supply roller 50a. An input gear 54 is freely rotatably mounted near the end of the rotary shaft 51, which gear is meshed with and rotated by a gear connected to a driving motor (not shown). Further, a cylindrical output boss 55 acting as a driving force transmitting member is inserted on and fixed to the end of the rotary shaft 51. The gear 54 and the output boss 55 have cylindrical portions 54a and 55a, respectively, which have substantially the same diameters and are disposed adjacent to each other. A clutch spring 56 is fitted on outer circumferential surfaces of the cylindrical portions 54a, 55a.
Further, a control cylinder 57 having a notch 57a formed therein is arranged around the clutch spring 56 and is press-fitted onto the cylindrical portions. The control cylinder has an integral projection 57b formed thereon, against which an actuator 58 driven by a solenoid (not shown) can be engaged or disengaged.
One end 56a of the clutch spring 56 is cocked in upright relation with the cylindrical portion 54a to engage the notch 57a of the control cylinder 57, and the other end 56b of the spring is fitted into a recess 55c formed in a flange portion 55b having a larger diameter than that of the cylindrical portion 55a of the output boss 55.
While the input gear 54 is rotated by the motor (not shown), since the actuator 58 is engaged by the projection 57b, the control cylinder 57 cannot be rotated, with the result that the input gear 54 is idly rotated due to the slip between the input gear 54 and the clutch spring 56.
When a sheet feed signal is given from a controller (not shown), the solenoid is energized to attract the actuator 58, thereby disengaging the latter from the projection 57b.
Accordingly, the clutch spring 56 is tightened against the cylindrical portions 54a, 55a by a tightening force thereof and a friction force between the spring and the input gear 54, whereby the clutch spring 56, output boss 55 and rotary shaft 51 are rotated altogether. As a result, the supply roller 50a fixed to the rotary shaft 51 is also rotated to feed the sheet material S.
However, in the above conventional technique, as shown in FIG. 13, since the one end 56a of the spring clutch 56 is engaged by the notch 57a of the control cylinder 57, the rotation force of the input gear 54 causes the inner diameter of the clutch spring 56 to increase, thus reducing the tightening force of the clutch spring 56 with respect to the cylindrical portion 54a. Consequently, the expansion of the inner diameter of the clutch spring 56 will be a maximum at its end near the input gear 54 and will gradually be decreased toward the other end.
In order to maintain the disengagement or non-connection condition of the clutch, a torque having a predetermined magnitude must be applied to the end 56a of the clutch spring 56. Since such torque is obtained not by an external force but by a friction force between the cylindrical portion 54a and the clutch spring 56, it is necessary that the clutch spring 56 be always slidingly contacted with the cylindrical portion 54a even when the clutch is in the disengagement condition.
As a result, there often occurs wear, scratch and/or noise due to the stick slip at portions shown by hatched lines in FIG. 13.
However, since the clutch spring 56 is fitted onto the cylindrical portion 54a, as shown in FIGS. 14A and 14B, when the clutch spring 56 is assembled on the cylindrical portion 54a, the lubricant 59 is locally retained at a surface area of the cylindrical portion 54a other than the rear where the clutch spring 56 contacts the cylindrical portion 54a, and, thus, is not supplied to the sliding portion between the clutch spring 56 and the cylindrical portion 54a.
Further, many of the used clutch springs comprise coils having rectangular cross-sections wound spirally in order to provide a transmission torque. However, since the edges of the rectangular coil or coils scrape or sweep the surface of the cylindrical portion 54a, if the lubricant 59 remains between the clutch spring and the cylindrical portion, it will be swept from the surface of the cylindrical portion 54a to worsen the lubrication feature between the clutch spring and the cylindrical portion.
On the other hand, when the cylindrical portion 54a is made of porous material to keep the lubricant into the minute holes on the cylindrical portion, since the cylindrical portion 54a has a rough surface, the contact area between the clutch spring 56 and the cylindrical portion 54a is decreased, with the result that the adequate transmission torque cannot be obtained.
Accordingly, in the past the lubricant 59 could not adequately be supplied to the sliding surface of the cylindrical portion 54a on which the clutch spring 56 was slidably moved.